1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for enabling information transmission at a maximum ratio from a Base Station (BS) to a Mobile Station (MS) in a BWA communication system using Relay Stations (RSs).
2. Description of the Related Art
Providing services with diverse Quality of Service (QoS) requirements at or above 100 Mbps to users is an active study area for a future-generation communication system called a 4th Generation (4G) communication system. Particularly, active research is being conducted on the provisioning of high-speed service by ensuring mobility and QoS to BWA communication systems such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN). Such major examples of these systems are Institute of Electrical and Electronics Engineers (IEEE) 802.16a and IEEE 802.16e.
The IEEE 802.16a and IEEE 802.16e communication systems adopt Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) to support a broadband transmission network for WMAN physical channels. IEEE 802.16a considers only a single-cell structure with no regard to mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports the SSs' mobility to the IEEE 802.16d communication system. A Mobile SS is referred to as an MS. Herein below, both the MS and the SS are commonly called user terminals.
FIG. 1 illustrates the configuration of a conventional IEEE 802.16e communication system.
In FIG. 1, the IEEE 802.16e communication system is configured in a multi-cell structure. Specifically, it includes cells 100 and 150, BSs 110 and 140 for managing the cells 100 and 150, respectively, and a plurality of user terminals 111, 113, 130, 151 and 153. Signaling is carried out in OFDM/OFDMA between the BSs 110 and 140 and the user terminals 111, 113, 130, 151 and 153.
The user terminal 130 is located in a cell boundary area between the cells 100 and 150, i.e. in a handover region. When the user terminal 130 moves to the cell 150 managed by the BS 140 during communications with the BS 110, a serving BS of the user terminal 130 changes from the BS 110 to the BS 140.
Since signaling is performed between a user terminal and a fixed BS via a direct link as illustrated in FIG. 1, a highly-reliable radio communication link can be established between them in the conventional IEEE 802.16e communication system. However, due to the fixedness of BSs, a wireless network cannot be configured with flexibility. As a result, the IEEE 802.16e communication system is not effective in efficiently providing communication services under a radio environment experiencing a fluctuating traffic distribution and a great change in the number of required calls.
The above problem can be solved by applying a multi-hop relay data transmission scheme using fixed Relay Stations (RSs), mobile RSs, or general user terminals to general cellular wireless communication systems such as IEEE 802.16e.
The multi-hop relay wireless communication system can advantageously reconfigure a network rapidly according to a communication environmental change and enables efficient operation of the whole wireless network. It can expand cell coverage and increase system capacity. When the channel status between a BS and a user terminal is bad, an RS can be installed between them so that the resulting establishment of a multi-hop relay path through the RS renders a higher-speed radio channel available to the user terminal. With the use of the multi-hop relay scheme at a cell boundary offering a bad channel status, high-speed data channels can be provided and the cell coverage area can be expanded.
FIG. 2 illustrates the configuration of a conventional relay BWA communication system.
Referring to FIG. 2, the multi-hop relay BWA communication system, which is configured in a multi-cell structure, includes cells 200 and 240, BSs 210 and 250 for managing the cells 200 and 240, respectively; a plurality of user terminals 211, 213, 251, 253 and 255 within the coverage areas of the cells 200 and 240; a plurality of user terminals 221, 223, 261 and 263 managed by the BSs 210 and 250 but located in areas 230 and 270 outside the cells 200 and 240; and RSs 220 and 260 for providing multi-hop relay paths between the BSs 210 and 250 and the user terminals 221, 223, 261 and 263.
OFDM/OFDMA signals are exchanged among the BSs 210 and 250, the RSs 220 and 260, and the user terminals 211, 213, 221, 223, 251, 253, 255, 261 and 263. Although the user terminals 211 and 213, within the coverage area of the cell 200, and the RS 220 can communicate directly with the BS 210, the user terminals 221 and 223, within the area 230, cannot directly communicate with the BS 210. Therefore, the RS 220, covering the area 230, relays signals between the BS 210 and the user terminals 211 and 223. That is, the user terminals 221 and 223 can transmit and receive signals to and from the BS 210 via the RS 220.
Meanwhile, although the user terminals 251, 253 and 255, within the coverage area of the cell 240, and the RS 260 can communicate directly with the BS 250, the user terminals 261 and 263, within the area 270, cannot directly communicate with the BS 250. Therefore, the RS 260 covering the area 270 relays signals between the BS 250 and the user terminals 261 and 263. That is, the user terminals 261 and 263 can exchange signals with the BS 250 via the RS 260.
In order to effectively deliver signals in the BWA communication system, diversity schemes have been studied. The diversity schemes aim to increase link reliabilities among an RS, a BS, and a user terminal and signal detectability at a receiver.
One of the diversity schemes, transmit diversity, is branched into Maximum Ratio Transmission (MRT) and Maximum Ratio Combining (MRC).
MRC uses a plurality of receive-antennas or a plurality of frequency channels, whereas MRT requires a plurality of transmit-antennas and channel state information feedback from a receiver to a transmitter. In the former scheme, signals received at the receive-antennas are combined to achieve a maximum Signal-to-Noise Ratio (SNR), resulting in weighting the received signals with conjugate channel gains. The latter scheme requires neither physical equipment such as a plurality of receive-antennas and nor additional radio resources to achieve a diversity gain. It also decreases inter-cell interference, thereby saving power from transmission.
However, MRT has been studied with no regard to use of RSs. Accordingly, with the introduction of a BWA communication system using RSs, there exists a need for developing an MRT scheme involving the RSs.